Commutators comprise a circular series of radially extending bars of predetermined length which are provided on one end of armatures of many types of electric motors and generators. The bars are insulated from each other by relatively thin sheets of mica. The overall exterior surface of a commutator of this type is substantially cylindrical and the inner ends of said bars are appropriately connected electrically to the windings of the armature. The commutators are engaged by carbon brushes which slidably engage the bars of the commutator in order to establish certain successive momentary electrical contacts therewith. The brushes normally are spring-pressed toward the commutator so that there is continual, gradual wearing of the outer surfaces of the commutator bars and the ends of the carbon brushes which engage the same.
When commutators are being manufactured, the outer surface thereof usually is machined to establish a uniform overall cylindrical configuration. At this stage, the outer edges of the mica insulation strips normally are coextensive with the outer surfaces of the commutator bars which usually are made of copper. It is necessary to cut away the outer edge portions of the mica strips to a predetermined depth because it is harder than the copper and causes undue wear of carbon brushes if they are permitted to engage the same. Such cutting away of the outer edges of the mica strips is known as undercutting the same. Such undercutting is performed on specialized machines adapted to that purpose and provided with narrow cutters capable of effectively cutting or milling the outer edges of the mica sheets.
Further, when commutators become worn to such extent that the outer edges of the mica insulating strips are substantially even with the worn outer surfaces of the copper bars, it is necessary to machine the commutators to reestablish a desired operative surface. When in such worn condition, it is not uncommon that the surfaces of the bars which are engaged by the carbon brushes actually have shallow grooves worn therein by said brushes and it is necessary to restore the outer surface of the commutator to a substantially cylindrical shape and thereby remove said grooved effects. This is done by machining the outer surface of the commutator in an appropriate cutting lathe and, as in regard to when the armature is newly manufactured, it then is necessary to undercut the insulating strips of mica to dispose the outer edges thereof at a level below the overall cylindrical outer surface of the commutator.
This invention pertains to the mica undercutting machines which perform the above outlined undercutting process.
It has been known in the prior art to drive an undercutting assembly along a shaft via a rack and pinion drive for a distance corresponding to the length of a mica slot to be undercut. Prior art systems have utilized limit switches to control the length of traverse of an undercutting saw. These switches can be in the form of mechanical limit switches or proximity sensors.
Since the proximity sensors are manually set in place, a possibility of error arises which may, for example, result in the saw running into the riser of the armature and consequent damage to the saw blade and spindle damage.
It has also been known to utilize an undercutting saw blade which is mounted on a floating spindle assembly to accommodate a particular problem in the art, namely the skew which sometimes exists in the mica slots. Under ideal manufacturing conditions, the mica slots would lie parallel to the axis of the armature and commutator. However, due to manufacturing inaccuracies and the difficulty of working with mica, the resulting mica slots may be skewed or out of alignment with the center line of the armature axis.
Prior art floating spindle assemblies were designed to accommodate the above mentioned skew of the mica slots by allowing the saw blade to float up and down and thereby stay within and appropriately undercut the mica slots.
However, if the skew is relatively large, as has been found in some instances in commutators for DC motors in the mass transit field, the floating spindle design cannot adjust for this larger skew and it becomes necessary to manually adjust the machine to undercut the skewed mica slots. Each time the machine is adjusted for skewed slots, it must be readjusted for automatic operation.
Accordingly, it is an object of the present invention to provide a drive means for an undercutting assembly which will not result in damage to the saw blade and spindle assembly should there be an inaccurate location of the proximity sensors or should there be a failure of the proximity sensors to effectively halt the traverse of the undercutting assembly.
It is a further object of the present invention to provide an automatic skew compensator attachment as a part of the floating spindle assembly which will enable an overall undercutter assembly to compensate for the large amount of mica slot skew present in some DC motor commutators.
These and other objects and advantages of the present invention will become apparent to those of skill in the art in the detailed description which follows.